This invention relates to a monostable multivibrator circuit in which a signal amplifying transistor operates in a non-saturated state, and also to an FM detector circuit which employs the monostable multivibrator circuit.
A monostable multivibrator circuit usually has a time constant circuit which uses a capacitor, an amplifier circuit which has an input threshold voltage and which is connected to the output end of the time constant circuit, a positive feedback circuit which is connected between the output end of the amplifier circuit and the input end of the time constant circuit, and a trigger terminal which is disposed in a predetermined portion of the circuit loop.
As regards a signal amplifying transistor used in the amplifier circuit or the positive feedback circuit, when the collector-emitter voltage has fallen below the saturation voltage, the current gain drops, and the change of the collector current responsive to the change of an input signal diminishes. Therefore, it becomes impossible to bestow a driving current of an abrupt change on a load connected to the transistor, and the rectangular waveform of an output signal from the circuit becomes rounded or obtuse. On account of the saturation, carriers are accumulated in the collector region and base region of the transistor. The accumulated carriers cause a comparatively long delay time in the output signal.
It is desirable that the obtuseness of the signal waveform as stated above be less and that the delay time be shorter.
In, for example, the so-called pulse count detector circuit wherein a pulse signal of a duty ratio corresponding to the frequency of an FM (frequency-modulated) signal is formed by the monostable multivibrator circuit and is smoothed thereby to obtain a demodulated signal, if the pulse signal is obtuse, the demodulated signal will not attain a level corresponding exactly to the particular duty ratio. If the delay time is long, the repeated operation of short periods will be hindered, and there will be no response to an FM signal at a high frequency.